The present invention relates to a system for driving an optical head for projecting a laser beam on a flat surface of an object such as, a glass original plate for an optical video disk, an IC wafer or the like and focusing the optical head onto the optical flat surface of the object.
An optical head is applied to a system for inspecting a defect occurred on an optical surface as shown in FIG. 1. This inspecting system comprises a head support 2 moved in an X-axis direction, a laser unit 4 provided in the head support 2, a polarizing beam splitter 6, a quarter-wave plate 8, a mirror 10, a photoelectric conversion detector 12, and an objective lens 16 of an optical head 14, and so forth. An object, e.g., a glass original plate 22 for a video disk is placed on a stage 20 moved in a Y-axis direction on a stage base 18. In such a system, a laser beam is emitted from the laser unit 4, and is focused through the polarizing beam splitter 6, the quarter-wave plate 8, the mirror 10 and the objective lens 16 on the surface 24 to the glass original plate 22. The reflected laser beam thus reflected from the surface 24 to be inspected is again introduced through the objective lens 16, the mirror 10, the quarter-wave plate 8 and the polarizing beam splitter 6 to the detector 12, which thus inspects a defect such as, a scratch, dust, or the like on the surface 24 from the intensity of the reflected laser beam. Accordingly it is required so as to adequately introduce the reflected laser beam to the optical head 14 that the focal point of the objective lens 16 must be accurately located on the surface 24. The conventional objective lens 16 has a fixed focal length, and hence the surface of the glass original plate 22 having various thickness, or of the IC wafer having a thickness thinner than the glass original plate 22 is feasibly displaced from the infinitesimal depth of focus of the objective lens 16 (for example, in case that the diameter of the spot of the laser beam is less than 2 .mu.m, the depth of the focus is 1 to 2 .mu.m), the consecutive inspection cannot be heretofore performed disadvantageously. It is considered in order to eliminate such disadvantage that the objective lens 16 is moved along the optical path of the laser beam to focus the objective lens onto the surface, but the optical axis of the objective lens 16 may be inclined during the movement of the objective lens, and the laser beam might not be projected along the optical axis.